Guided Wave Testing
While traditional inspection methods provide value to plant operators, many have begun condition based inspection programs aimed at bolstering their mechanical integrity and preventive maintenance programs. With this in mind, many plants have chosen to inspect their piping systems using Guided Wave UT (GUL) Inspection. The benefits of this inspection include:
- Large sections of piping can be inspected rapidly and safely often with no scaffolding.
- Minimal insulation removal for Corrosion Under Insulation (CUI) inspections.
- Localized damage can be pinpointed and characterized as to length and depth.
- GUL is an excellent tool for cased crossings and un-piggable pipe.
- Point of contact corrosion can be found without lifting the pipe thus avoiding potential leaks and protecting the environment.
Ultrasonic phased array systems can potentially be employed in almost any test where conventional ultrasonic flaw detectors have traditionally been used. Weld inspection and crack detection are the most important applications, and these tests are done across a wide range of industries including aerospace, power generation, petrochemical, metal billet and tubular goods suppliers, pipeline construction and maintenance, structural metals, and general manufacturing.
The benefits of phased array technology over conventional UT come from its ability to use multiple elements to steer, focus and scan beams with a single transducer assembly. Beam steering, commonly referred to sectorial scanning, can be used for mapping components at appropriate angles. This can greatly simplify the inspection of components with complex geometry. The small footprint of the transducer and the ability to sweep the beam without moving the probe also aids inspection of such components in situations where there is limited access for mechanical scanning. Sectorial scanning is also typically used for weld inspection. The ability to test welds with multiple angles from a single probe greatly increases the probability of detection of anomalies. Electronic focusing permits optimizing the beam shape and size at the expected defect location, as well as further optimizing probability of detection. The ability to focus at multiple depths also improves the ability for sizing critical defects for volumetric inspections. Focusing can significantly improve signal-to-noise ratio in challenging applications, And electronic scanning across many groups of elements allows for C-Scan images to be produced very rapidly.
Time of Flight Diffraction (TOFD)
Time of Flight Diffraction or TOFD is one of the most promising ultrasonic techniques for the examination of welds on pressure vessels in lieu of radiography; for pipe weld quality or crack detection and also weld root erosion. TOFD is a computerized ultrasonic system able to scan, store, and evaluate indications in terms of height (through wall thickness), length and position, with a degree of accuracy and speed never achieved with other ultrasonic techniques. Time of Flight Diffraction (TOFD) is one of the methods used for flaw detection and sizing. Of all the ultrasonic crack sizing methods, TOFD is the most accurate methods. The inspection employs two longitudinal wave (L-wave) angle beam transducers arranged symmetrically opposite facing each other, straddling the weld or base material under test. One probe acts like a transmitter of ultrasonic energy while the other probe receives the ultrasound energy. The transducer, pulser, and amplifier characteristics are selected to generate as broad distribution of energy as possible over the material under test providing full weld coverage. A single-axis scan (that is, along the weld), with a position encoder records the position of the weld and enables the display of digital images in real time.
Magnetism, the underlying principle behind electric motors and generators, relays and stereo speakers, is also the force that enables an important category of NDT tools called eddy current instruments. Eddy current testing is widely used in the aerospace industry and in other manufacturing and service environments that require inspection of thin metal for potential safety-related or quality-related problems. In addition to crack detection in metal sheets and tubing, eddy current can be used for certain metal thickness measurements such as identifying corrosion under aircraft skin, to
measure conductivity and monitor the effects of heat treatment, and to determine the thickness of nonconductive coatings over conductive substrates. Both field portable and fixed system instruments are available to meet a wide variety of test needs.
Eddy current NDT can examine large areas very quickly, and it does not require use of coupling liquids. In addition to finding cracks, eddy current can also be used to check metal hardness and conductivity in applications where those properties are of interest, and to measure thin layers of nonconductive coatings like paint on metal parts. At the same time, eddy current testing is limited to materials that conduct electricity and thus cannot be used on plastics. In some cases, eddy current and ultrasonic testing are used together as complementary techniques, with eddy current having an advantage for quick surface testing and ultrasonic having better depth penetration.
Acoustic Emission (AE) is a nondestructive testing technique used in monitoring structures or components for flaws. This is done by the mechanically loading the test piece which forces the flaw to release energy. The energy that is release are stress waves carried along the surface of the structure and detected by AE sensors. AE has been used in the refining industry, chemical industry, power generation, manufacturing, pipelines and paper mills.
Some Benefits of AE
- AE is a global monitoring technique. The entire structure can be monitored from a few locations.
- Detects only active defects, not dormant or inactive defects.
- No vessel entry is needed.
- Most of the time, the equipment can be monitored while in service with little interruption to the normal process.
- Typically less time and less cost than other NDE methods due to testing a structure during only one test. Insulation in small areas only needs to be removed.
- If areas are detected in the AE testing, only those areas need further inspection.
Acoustic emission testing has been used on many types of vessels and components:
- Metal pressure vessels – piping, reactors, spheres
- Storage tanks
- Fiberglass tanks and vessels
- Hot reheat steam lines
- Tube trailers
- New vessels
Several loading techniques are used and the one of choice is over-pressurizing on-line. Other loading methods are pneumatic, hydrostatic, and thermal – including monitoring as a high temperature unit comes off line, leak testing, continuous on-line and remote continuous on-line.